GH-105900: Fix pathlib.Path.symlink_to(target_is_directory=...) docs (#114035)

Clarify that *target_is_directory* only matters if the target doesn't
exist.

Author: barneygale

Doc/library/pathlib.rst

@@ -1539,9 +1539,13 @@ call fails (for example because the path doesn't exist).
 
 .. method:: Path.symlink_to(target, target_is_directory=False)
 
-   Make this path a symbolic link to *target*.  Under Windows,
-   *target_is_directory* must be true (default ``False``) if the link's target
-   is a directory.  Under POSIX, *target_is_directory*'s value is ignored.
+   Make this path a symbolic link pointing to *target*.
+
+   On Windows, a symlink represents either a file or a directory, and does not
+   morph to the target dynamically.  If the target is present, the type of the
+   symlink will be created to match. Otherwise, the symlink will be created
+   as a directory if *target_is_directory* is ``True`` or a file symlink (the
+   default) otherwise.  On non-Windows platforms, *target_is_directory* is ignored.
 
    ::